<!DOCTYPE html>
<html>
<head>
    <meta charset="utf-8" />
    <title>Gamedev Workshop - lesson 2: move the ball</title>
    <style>* { padding: 0; margin: 0; } canvas { background: #eee; display: block; margin: 0 auto; }</style>
</head>
<body>

<canvas id="myCanvas" width="480" height="320"></canvas>

<script>
    var canvas = document.getElementById("myCanvas");
    var ctx = canvas.getContext("2d");
    var x = canvas.width/2;
    var y = canvas.height-30;
    var dx = 2;
    var dy = -2;

    function draw() {
        ctx.clearRect(0,0,480,320);
        ctx.beginPath();
        ctx.arc(x, y, 10, 0, Math.PI*2, true);
        ctx.fillStyle = "#009900";
        ctx.fill();
        ctx.closePath();
        x += dx;
        y += dy;
    }
    
    setInterval(draw, 10);
</script>

</body>
</html>